Leukemia is a cancer involving bone marrow, circulating blood cells, and organs such as the spleen and lymph nodes. It is usually characterized by an abnormal proliferation of leukocytes. Leukemias are classified as either acute or chronic based on cellular maturity. Acute leukemias consist of predominantly immature, poorly differentiated cells; chronic leukemias have more mature cells. Acute leukemias are divided into lymphocytic (ALL) and myelocytic (AML) types. Chronic leukemias are described as lymphocytic (CLL) or myelocytic.
ALL is the most common pediatric cancer; it also strikes adults of all ages. Malignant transformation and uncontrolled proliferation of an abnormally differentiated, long-lived hematopoietic progenitor cell result in a replacement of normal marrow by malignant cells, and the potential for leukemic infiltration of the central nervous system and abdominal organs. Symptoms include fatigue, pallor, infection, and easy bruising and bleeding. Examination of peripheral blood smear and bone marrow is usually diagnostic. Treatment typically includes combination chemotherapy to achieve remission, intrathecal chemotherapy for CNS prophylaxis and/or cerebral irradiation for intracerebral leukemic infiltration, consolidation chemotherapy with or without stem cell transplantation, and maintenance chemotherapy for 1 to 3 years to avoid relapse.
Two thirds of all ALL cases occur in children, with a peak incidence at age 2 to 10; ALL is the most common cancer in children and the second most common cause of death in children <age 15. A second rise in incidence occurs with aging after age 45.
The most common type of leukemia in the Western world, CLL involves mature-appearing defective neoplastic lymphocytes with an abnormally long life span. The peripheral blood, bone marrow, spleen, and lymph nodes undergo leukemic infiltration. Symptoms may be absent or may include lymphadenopathy, splenomegaly, hepatomegaly, and nonspecific symptoms attributable to anemia (fatigue, malaise). Diagnosis is by examination of peripheral blood smear and bone marrow aspirate. Treatment, delayed until symptoms develop, is aimed at lengthening life and decreasing symptoms and may involve chlorambucil or fludarabine, prednisone, cyclophosphamide, and/or doxorubicin. Monoclonal antibodies, such as alemtuzumab and rituximab, are increasingly being used. Palliative radiation therapy is reserved for patients whose lymphadenopathy or splenomegaly interferes with other organs.
ALL and CLL can each be subdivided into B-cell or T-cell leukemia. In certain cases, the clonal expansion is T cell in type, and even this group has several subtypes (eg, large granular lymphocytes with cytopenias). Chronic leukemic patterns categorized under CLL include prolymphocytic leukemia, leukemic phase of cutaneous T-cell lymphoma (ie, Sézary syndrome), hairy cell leukemia, and lymphoma leukemia (ie, leukemic changes seen in advanced stages of malignant lymphoma). Differentiation of these subtypes from typical CLL is usually straightforward.
As for ALL, a subcategory of ALL is Adult T cell leukemia (ATL), which is usually a highly aggressive non-Hodgkin's lymphoma. T-cell-prolymphocytic leukemia (T-PLL) is a mature T-cell leukemia with aggressive behavior and predilection for blood, bone marrow, lymph nodes, liver, spleen, and skin involvement. T-PLL primarily affects adults over the age of 30. T-cell acute lymphoblastic leukemia (T-ALL) is another type of ALL that mainly affects children and adolescents. This aggressive tumor is linked with a poor prognosis, resulting in rapid fatality in the absence of treatment. Current therapy for T-ALL requires multi-agent combination chemotherapy with long-term survival rate of only 30-40% among patients under 60 years of age.
Signaling pathways has been investigated in the context of T-cell maturation and malignant transformation. Certain genetic mutations found in ALLs highlight the importance of pre-TCR signaling and Notch signaling in leukemias.
The progressive maturation of αβ T cells in the thymus is a highly ordered process broadly characterized by the expression of CD4 and CD8 co-receptors. Early T cell progenitors are double negative (DN) for both CD4 and CD8, which can be further subdivided based on the unique expressions of CD44 and CD25 in the following order of development: DN1 (CD44+ CD25−), DN2 (CD44+ CD25+), DN3 (CD44− CD25+), and DN4 (CD44− CD25−) (Godfrey et al., 1993). At the DN2 stage, the rearrangement of the T-cell receptor (TCRβ) locus is initiated by recombinase-activating gene 1 (RAG-1) and RAG-2 and proceeds until a functional TCRβ chain is generated at the DN3 stage (Capone et al., 1998; Godfrey et al., 1994; Livak et al., 1999). In DN3 cells, only the productively rearranged TCRβ are allowed to pair with invariant pre-Tα and CD3 molecules to form a pre-TCR complex, signifying the completion of the first critical checkpoint during T cell development, known as β-selection (Dudley et al., 1994; Mallick et al., 1993). The ensuing signals from the pre-TCR complex act to promote survival and expansion of DN3 thymocytes, terminate further rearrangement of the TCRβ locus, and induce differentiation into the DP stage (Michie and Zuniga-Pflucker, 2002; von Boehmer et al., 1999). As early thymic progenitors exit the DN stage, they acquire the expression of both CD4 and CD8 to become double positive (DP) thymocytes and a select few will mature into CD4 or CD8 single positive (SP) cells.
In addition to the autonomous signals received through the pre-TCR, extrinsic signals derived form the thymic microenvironment are also fundamental for the proper development of T cells. In particular, signaling through Notch receptors in common lymphoid progenitors that had colonized the thymus have been reported to influence the T/B cell lineage decision by suppressing B cell development and promoting T cell commitment (Pui et al., 1999; Radtke et al., 1999). Furthermore, αβ-committed progenitors, which are first evident at the DN2 stage, rely on Notch signals for survival signals prior to TCRβ expression (Ciofani et al., 2006). Following β-selection, αβ-committed progenitors may continue to depend on Notch signals, which synergize with pre-TCR to induce expansion and differentiation during the DN3 to DP transition (Garbe and von Boehmer, 2007; Guidos, 2006). The importance of Notch activity for T cell commitment and differentiation throughout DN stages was particularly evident as OP9 stromal cell line ectopically expressing the Notch ligand Delta-like 1 (DL1) was shown to support T cell differentiation from hematopoietic progenitors (Schmitt and Zuniga-Pflucker, 2002).
Although Notch and pre-TCR signaling pathways are both involved in T-cell maturation and cell expansion, there is currently no known effective therapy against leukemia that take into account the cooperativity between the two pathways. Methods of regulating multiple pathways relevant in cellular expansion and maturation are of great interest for clinical and research purposes.
Small regulatory RNAs mediate a fundamental layer of gene regulation known as RNA interference. These tiny fragments of nucleic acid can globally affect gene expression, and in turn, alter developmental processes in plants and animals (Ambros, 2003; Bartel, 2004). Among these small non-coding RNAs, microRNAs (miRNAs) represent a family of naturally occurring RNA molecules of ˜22 nucleotides in length that mediate posttranscriptional gene repression by binding with imperfect complementarity to the 3′ untranslated region (3′UTR) of their cognate target messenger RNA (mRNA), resulting in message degradation or translational repression (Pillai et al., 2007). mRNA genes are abundant in nature and computational prediction suggests that at least one-third of all human protein-coding genes are regulated by miRNAs (Berezikov et al., 2005; Miranda et al., 2006). Their impressions in mammals have also been appreciated as miRNAs were shown to regulate insulin secretion (Poy et al., 2004), adipocyte differentiation (Esau et al., 2004), and heart development (Zhao et al., 2007; Zhao et al., 2005).
Chen et al. (2004) Science 303:83 describe the modulation of hematopoietic lineage differentiation by microRNAs. Krutzfeldt et al. (2005) Nature 438:685 describe the silencing of microRNAs in vivo with antagomirs.
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